1. Field of the Invention
This invention generally relates to the testing of underground formations or reservoirs. More particularly, this invention relates to a method and apparatus for real-time closed-loop control of a draw down system.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. A large portion of the current drilling activity involves directional drilling, i.e., drilling boreholes deviated from vertical and/or horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from earth formations. Modern directional drilling systems generally employ a drill string having a bottom hole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of down hole devices placed in close proximity to the drill bit measure certain down hole operating parameters associated with the drill string. Such devices typically include sensors for measuring down hole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional down hole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations.
One type of while-drilling test involves producing fluid from the reservoir, collecting samples, shutting-in the well, reducing a test volume pressure, and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different reservoirs within a given borehole or at several points in a single reservoir. This type of test is known as a “Pressure Build-up Test.” One important aspect of data collected during such a Pressure Build-up Test is the pressure build-up information gathered after drawing down the pressure in the test volume. From this data, information can be derived as to permeability and size of the reservoir. Moreover, actual samples of the reservoir fluid can be obtained and tested to gather Pressure-Volume-Temperature data relevant to the reservoir's hydrocarbon distribution.
Some systems require retrieval of the drill string from the borehole to perform pressure testing. The drill string is removed, and a pressure measuring tool is run into the borehole using a wireline tool having packers for isolating the reservoir. Although wireline conveyed tools are capable of testing a reservoir, it is difficult to convey a wireline tool in a deviated borehole.
A more recent MWD system is disclosed in U.S. Pat. No. 5,803,186 to Berger et al. The '186 patent provides a MWD system that includes use of pressure and resistivity sensors with the MWD system, to allow for real time data transmission of those measurements. The '186 device enables obtaining static pressures, pressure build-ups, and pressure draw-downs with a work string, such as a drill string, in place. Also, computation of permeability and other reservoir parameters based on the pressure measurements can be accomplished without removing the drill string from the borehole.
Using a device as described in the '186 patent, density of the drilling fluid is calculated during drilling to adjust drilling efficiency while maintaining safety. The density calculation is based upon the desired relationship between the weight of the drilling mud column and the predicted down hole pressures to be encountered. After a test is taken a new prediction is made, the mud density is adjusted as required and the bit advances until another test is taken.
A drawback of this type of tool is encountered when different formations are penetrated during drilling. The pressure can change significantly from one formation to the next and in short distances due to different formation compositions. If formation pressure is lower than expected, the pressure from the mud column may cause unnecessary damage to the formation. If the formation pressure is higher than expected, a pressure kick could result.
Such formation pressure testing can be hampered by a variety of factors including insufficient draw down volume, tool or formation plugging during a test, seal failure, or pressure supercharging. These factors can result in false pressure information. Pressure tests with excessive draw rate, i.e. the rate of volume increase in the system, or tests with an insufficient draw volume should be avoided. The excessive draw rate often results in an excessive delta pressure drop between the test volume and the formation causing long build up times. Moreover, compressibility of fluid in the tool will dominate the pressure response if the formation cannot provide enough fluid for the excessive pressure drop. With an excessive draw rate the pressure drop can exceed the fluid bubble point thereby causing gas to evolve from the fluid and corrupt the test result.
With insufficient draw down volume pressure in the tool will not fall below the formation pressure resulting in little or no pressure build up. In very permeable formations, insufficient draw down volume can falsely indicate a tight formation.
Pressure supercharging, or simply supercharging, exists when pressure at the sandface near the borehole wall is greater than the true formation pressure. Supercharging is caused by fluid invasion from the drilling process that has not completely dissipated into the formation. Supercharging is also caused by annulus fluid pressure bypassing a seal through the mudcake. Consequently, measured pressure information is typically measured more than once to provide verification of the information.
The typical verification test involves multiple draw down tests where using identical draw down parameters, e.g. draw rate, delta pressure and test duration. In some cases, the parameters might be varied according to a predetermined verification protocol. The multiple draw test using the same test parameters suffers from inefficiency of time and the possibility of repeating erroneous results. Merely following a predetermined test protocol does not increase efficiency, because the protocol might not address real-time conditions in a timely manner. Furthermore, predetermined protocols will not necessarily verify previous test results.
A common practice is to set a fixed draw down rate, also referred to as draw rate. Setting a fixed draw rate results in an uncontrolled transition from zero rate to the set fixed draw rate. The common tool also instantaneously halts the draw portion of the test after a predetermined time period, thereby creating another uncontrolled transition from the fixed rate back to zero, these uncontrolled transitions result in discontinuities at the transition points, which are not well followed by test equipment and sensors, particularly pressure sensors used in down hole applications.
The combination of discontinuities created by current test procedures coupled with the typical sensor response results in several deficiencies. The pressure sensor output signal will typically lag behind the actual pressure existing in the test volume. Sometimes the pressure sensor will “overshoot” by indicating a pressure beyond (higher or lower) than the actual limit pressure. The abrupt transitions will also alter the test environment causing erroneous pressure measurements. The transition points result in a relatively quick pressure change causing a temperature change. When there is a high pressure gradient, the temperature change will be even greater resulting in poor temperature equalization, which will lead to incorrect pressure measurements with the typical temperature-compensated pressure sensors. When these deficiencies are present, analytical methods of determining formation parameters such as pressure, mobility and compressibility are inaccurate, and even direct measurement of formation pressure is inaccurate.
Any of the above identified problems can lead to false information regarding formation properties and to wasted rig time. Therefore, there is a need to provide a method and apparatus for performing multiple verification tests without operator intervention. Furthermore, there is a need to provide an apparatus and method for a smooth transition from a zero draw-rate to a set maximum draw-rate and then for a smooth transition back to zero draw rate.